1. Field of the Invention
The present invention relates to a microprobe chip for detecting evanescent waves which is used in near-field scanning optical microscopes and a method for making the same, a probe including a thin film cantilever provided with the microprobe chip and a method for making the same, an evanescent wave detector, a near-field scanning optical microscope, and an information regenerator provided with the microprobe chip. In particular, the present invention relates to a microprobe chip having a tip with a small curvature, which is suitable for these apparatuses, and a method for making the same which is capable of producing the microprobe chip with high productivity.
2. Related Background Art
A scanning tunnel microscope (hereinafter referred to as an STM) was developed by G. Binning et al. in 1983 (Phys. Rev. Lett., 49, 57 (1983)). The STM can directly observe electronic structures of surface atoms on conductive materials, such as single crystals and amorphous materials and can obtain real space images with high resolution. Thus, various scanning probe microscopes (hereinafter referred to as SPMs) have been intensively investigated in microstructure analysis of materials.
Examples of SPMs include scanning tunnel microscopes, atomic force microscopes (AFMs), magnetic force microscopes (MFMs), and near-field scanning optical microscopes (NSOMs), which detect the surface structure of a material by means of changes in tunnel currents, atomic forces, magnetic forces, and light intensities, respectively. Such changes occur when scanning near the surface of the material with probes provided with microprobe chips.
Among these SPMs, NSOMs permit nondestructive measurement of fine patterns on tested materials with high resolution, that is, a positional resolution of less than .lambda./2, which has not been achieved by conventional optical microscopes, by using evanescent light radiated from a fine pinhole. Further, NSOMs are applicable to various materials which have not been observed by any conventional method, such as organisms and biological cells.
The evanescent waves are detected by the following three methods.
The first method was developed by E. Betzig, et al. ("Collection Mode Near-Field Scanning Optical Microscopy", Appl. Phys. Lett. 51(25), pp. 2088-2090 (1987)). Illuminating light is incident on the back surface of a test piece so as to satisfy the total reflection condition, and the evanescent waves occurring on the front surface of the test piece due to the illuminating light are detected with a microprobe chip provided with a fine aperture. This method is capable of obtaining evanescent wave images with high resolution, and thus has been most intensively studied.
The microprobe chip is composed of a glass pipette or optical fiber of which the tip is pointed. It is therefore fabricated by mechanical polishing or the like, with low productivity and high production costs. Further, the aperture is hardly ever formed with satisfactory reproducibility and high accuracy.
The second method uses a thin film cantilever composed of a silicon nitride thin film used in AFMs instead of the aperture to detect the scattered light of evanescent waves (N. F. van Hulst, et al., "Near-Field Optical Microscope Using a Silicon-Nitride Probe", Appl. Phys. Lett. 62(5), pp. 461-463 (1993)).
A typical microprobe chip used in the second method and a method for making the microprobe chip are disclosed in U.S. Pat. No. 5,221,415, in which the microprobe chip is formed by anisotropic etching of single-crystal silicon in the crystal axes by means of a semiconductor production process. As shown in FIG. 1, a pit 518 is formed on a silicon wafer 514 covered with silicon dioxide masks 510 and 512 by an anisotropic etching process, the silicon dioxide masks 510 and 512 are removed, and then the silicon wafer 514 is covered with silicon nitride layers 520 and 521. The silicon nitride layer 520 has a pyramidal pit 522 directly on top of the pit 518. After the silicon nitride layer 521 on the bottom surface is removed, a glass plate 530 provided with a sawcut 534 and a Cr layer 532 is joined to the silicon nitride layer 520. The silicon wafer 514 is removed by etching. As a result, a probe consisting of a microprobe chip and a cantilever which are composed of silicon nitride is replicated on a mounting block. When the probe is used in an optical lever-type AFM, a metal film 542 as a reflecting film is formed on the bottom surface. The probe can be produced with high productivity and reproducibility and has a pointed tip. The probe, however, forms a lower resolution NSOM image than that formed by a probe with an aperture produced by the first method.
In the first and second methods, the microprobe chip is used as an optical pickup and the scattered evanescent-wave light is amplified by a photomultiplier cell provided above the microprobe chip. On the other hand, the third method involves direct detection of scattered evanescent-wave light using a photodiode on a thin film cantilever (S. Akamine, et al., "Development of a Microphotocantilever for Near-Field Scanning Optical Microscopy", Proceedings of the IEEE MicroElectro Mechanical Systems Workshop 1995, pp. 145-150). FIG. 2 is a cross-sectional view of a microprobe chip produced by the third method. The microprobe chip consists of a p-silicon thin film cantilever 601 of which one end is supported by a silicon substrate 600, a photodiode of pn junction 603 formed by providing an n layer 602, a silicon oxide film 604 provided thereon, and an aluminum wiring layer 605 provided on the silicon oxide film 604 which extracts scattered light signals from the photodiode. The lower face of the thin film cantilever is provided with an etch stop layer 606 which is used for producing the cantilever.
It is possible for the photodiode optical detector provided on the free end of the cantilever to approach the test piece, and hence the SN ratio and resolution can be improved. Further, the photodiode optical detector can simplify the system configuration. In the third method, however, the thin film cantilever, as a microprobe chip, is produced by a photolithographic process and an etching process with poor reproducibility, and hence microprobe chips having the same shape cannot be produced in the same production lot.